1. Field of the Invention
The present invention relates to a high frequency oscillator for microwave and millimeter bands, and more particularly, to a high frequency oscillator which employs a coplanar line based planar resonator circuit.
2. Description of the Related Arts
A high frequency oscillator for a microwave band or a millimeter band is a highly significant component in terms of performance as well as cost even among main components used in a high frequency apparatus or a high speed apparatus. A high frequency oscillator of the type mentioned above employs a planar resonator circuit of a microstrip line type. FIG. 1A is a plan view illustrating an example of such a conventional high frequency oscillator, and FIG. 1B is a cross-sectional view taken along a line A—A in FIG. 1A.
The high frequency oscillator illustrated in FIGS. 1A and 1B comprises microstrip line resonator circuit 1, and a negative resistance element (i.e., oscillation element), for example, Gunn diode 2 which is a two-port element. Resonator circuit 1 comprises substrate 3 made of a dielectric material or the like; circular circuit conductor 1A formed on one principal surface of substrate 3; and ground conductor 4 formed substantially over the entirety of the other principal surface of substrate 3. On the one principal surface of substrate 3, matching line 5 is extended from one end (left end in FIG. 1A) on the periphery of circuit conductor 1A, and one end of Gunn diode 2 is connected to matching line 5. The other end of Gunn diode 2 is connected to ground conductor 4 through a via hole (not shown) formed in substrate 3. Output line 6 in a microstrip line structure is also connected to another end (right end in FIG. 1A) on the periphery of circuit conductor 1A, and another capacitive matching line 5 is connected to output line 6.
In the illustrated high frequency oscillator, Gunn diode 2 presents a negative resistance in a resonant frequency region of microstrip line resonator circuit 1. Resonator circuit 1 is powered by Gunn diode 2 to maintain a resonant state, resulting in oscillation. The oscillation frequency generally depends on the resonant frequency of resonator circuit 1 which is determined by the size of circuit conductor 1A and the dielectric coefficient of substrate 3. It should be noted that discrete elements such as Gunn diode 2 are much expected as oscillation elements, particularly in a millimeter band and the like, because they are relatively inexpensive.
However, in the high frequency oscillator configured as described above, the negative resistance element such as a Gunn diode has relatively low impedance, while the microstrip line resonator circuit has high impedance, so that it is generally difficult to match the impedance between the two components. In addition, the substrate must be provided with a via hole for grounding the Gunn diode in high frequency terms, causing deteriorated characteristics resulting from a parasitic reactance of the via hole, for example, a reduction in oscillation output (or oscillation power) associated with a circuit loss, particularly when the oscillation frequency is high. Moreover, the requirement for a process for forming the via hole impedes an improved productivity.
A high frequency oscillator proposed for solving problems such as a reduction in oscillation power utilizes a higher order resonance mode (for example, TM21) of microstrip line resonator circuit 1, and Gunn diodes 2 connected through matching lines 5 to a plurality of equipotential points located at geometrically symmetric points on resonator circuit 1, as illustrated in FIG. 2, to combine oscillation outputs. However, even this high frequency oscillator needs a via hole and accordingly cannot avoid the influence of the reactance which is parasite on the via hole, resulting in a lower combination efficiency of the oscillation power.